In recent years, electric motor-driven equipment that include mobile electronic equipment such as mobile computers, digital cameras, etc. for which compaction of size and weight is demanded tend to rapidly increase. As a power supply of such electric motor-driven equipment, a sealed nickel metal-hydride storage battery is more widely used than a nickel cadmium storage battery, lead storage battery, etc., because the former provides a higher energy density per unit volume or unit weight, has a higher resistance to over-charge and higher resistance to over-discharge, and are less harmful to the environment than the latter. The application field of a sealed nickel metal-hydride storage battery has spread rapidly and it is now used as a power source of hybrid electric vehicles (HEVs), and even of electric motor-driven tools and electric toys which require high output power performance (high-rate discharging ability) from batteries to supply power and which have been driven heretofore by nickel cadmium batteries.
However, the aforementioned hydrogen absorbing alloy has a number of drawbacks: it is vulnerable to corrosion due to electrolyte; the hydrogen absorbing electrode (negative electrode) has a poorer high-rate discharging ability and charge receptivity than a nickel electrode (positive electrode), and thus to maintain a balance against the positive electrode, the negative electrode must comprise a hydrogen absorbing alloy having a larger volume (about 1.5 time) than that of the positive electrode, which makes it difficult to raise the energy density of the negative electrode. The hydrogen absorbing alloy, though being highly resistant to corrosion and having a long life, is slow in activation and, if it is used neat as an electrode, requires a considerable time for initial activation before it exhibits a sufficiently high discharging activity. Specifically, it requires several tens charge/discharge cycles, or in some cases even several hundreds charge/discharge cycles, before it becomes sufficiently active.
To be used as a power supply of HEVs, electric motor-driven tools and electric motor-driven toys, it is necessary for a nickel metal-hydride battery to have a better charge/discharge cycle performance and high-rate discharging ability.
To solve a problem involved in the delayed activation of a hydrogen absorbing alloy, although it is highly resistant to corrosion, many remedial methods have been proposed for promoting the activity of a hydrogen absorbing alloy powder or hydrogen absorbing alloy electrode. One of such methods is to subject a hydrogen absorbing alloy powder to a surface treatment which consists of immersing the powder in an acidic aqueous solution having a pH of 0.5 to 3.5 (for example, see Japanese Patent Application Publication No. 7-73878 (page 3, paragraph 0011)).
According to this patent document, the acidic treatment removes the coat of oxides or hydroxides covering the surface of particles of a hydrogen absorbing alloy powder, and recovers the clean surface of the powder, which enables the improved activity of the hydrogen absorbing electrode, and reduces the time necessary for activation. However, this treatment is not as effective for improving the cycle life of the battery. This is probably because, since elements eluted by the acidic treatment are different from the elements eluted in an aqueous solution of alkali metal which serves as an electrolyte of a nickel metal-hydride battery, the hydrogen absorbing alloy powder is corroded by the alkaline solution when the hydrogen absorbing alloy powder treated by the acidic treatment is used in the construction of a nickel metal-hydride storage battery.
Another method consists of immersing a hydrogen absorbing alloy powder in an aqueous sodium hydroxide solution containing sodium hydroxide at a concentration of 30 to 80 wt % at a temperature equal to or higher than 90° C., thereby producing an alloy powder which has a high-rate discharging ability and cyclic performance and is suitable for electrode use (for example, see Japanese Patent Application Publication No. 2002-256301 (page 3, paragraph 0009)).
According to the above-cited patent document, treatment with an aqueous conc. NaOH solution at a high temperature can remove oxides attached to the surface of the material powder more efficiently in a shorter period than does the treatment using a KOH aqueous solution. Furthermore, the treatment impedes the fresh attachment of oxides to the exposed surface of the alloy powder, thereby reducing the contact resistance of the powder, and improving its reactivity. Thus, according to this patent document, the time spent for procedures necessary for activating a hydrogen absorbing alloy is reduced, and discharging ability that is excellent from the initial phase of charge/discharge cycles is obtained, but the cycle performance is still inadequate. The hydrogen absorbing electrode produced by the method has a high-rate discharging ability better than a conventional comparable hydrogen absorbing electrode, but its high-rate discharging ability does not necessarily reach a level sufficiently high to meet the stern standard sought by hybrid electric vehicles (HEVs), electric motor-driven tools, etc.
According to the last-mentioned patent document, a hydrogen absorbing alloy is immersed in an alkaline aqueous solution before it is installed in a battery, so as to allow, on the surface of the hydrogen absorbing alloy, a layer to be formed which is stable to the alkaline aqueous solution, so that the hydrogen absorbing alloy powder can be protected against corrosion when it is installed in a battery. However, when the storage battery undergoes charge/discharge cycles, the alloy experiences a series of cycles consisting of hydrogen absorption and hydrogen desorption, and in conjunction with these cycles, the alloy repeats expansion/shrinkage which causes the alloy powder to have strain, and breaks the powder into finer particles. Therefore, as a result of the repeated charge/discharge cycles, the alloy exposes its fresh surfaces which are then exposed to the electrolyte to be corroded by the latter, which will lead to the reduction of charge reserve capacity. Because of this, the method described in the second patent document is not likely to bring about a significant improvement in the cycle performance. Furthermore, the repetition of charge/discharge cycles leads to the rapid decline of high-rate discharging ability. This is probably based on the following mechanism: when the battery undergoes a series of charge/discharge cycles, lighter rare earth elements such as La and the like, and Mn and Al contained in the hydrogen absorbing alloy are eluted although small in amount, to deposit, as hydroxides, on the surface of hydrogen absorbing alloy powder, which interferes with electrode reactions.
A third production of a hydrogen absorbing electrode is proposed which consists of adding, prior to the preparation of a hydrogen absorbing electrode, an yttrium (Y) compound or a compound of a lighter rare earth element such as lanthanum (La), cerium (Ce), praseodymium (Pr), etc. to a hydrogen absorbing alloy powder, in order to enhance the resistance to corrosion of the hydrogen absorbing alloy powder while maintaining the output power performance of the hydrogen absorbing electrode (see, for example, Japanese Unexamined Patent Application Publication No. 11-260361).
However, a sufficient life-improving effect could not be obtained particularly at high temperature probably because the enhanced corrosion resistance conferred by the yttrium compound to the hydrogen absorbing alloy powder may not be sufficiently high, or probably because the corrosion resistance enhancing effect of the yttrium compound may be impaired by a lighter rare earth element such as La used in combination. In any case, the aforementioned means did not bring about a battery possessed of an excellent high-rate discharging ability and a long cycle life.
A fourth hydrogen absorbing electrode has been proposed which is obtained by immersing, in advance, a hydrogen absorbing alloy powder in an alkaline or weakly acidic aqueous solution, and adding, to the powder, a rare earth element which is less basic than La, such as Sm, Gd, Ho, Er, Yb, etc. neatly or in the form of a compound for mixture (see, for example, U.S. Pat. No. 6,136,473 and Japanese Patent Application Publication No. 9-7588).
According to the description given in those patent documents, it is possible to protect a hydrogen absorbing alloy powder against corrosion which would otherwise result during its immersion in alkaline electrolyte, and to enhance its durability by depositing the hydroxides or oxides of a rare earth element as described above on the surface of the hydrogen absorbing alloy. However, when the method is actually practiced, it is found in some cases that the internal resistance of hydrogen absorbing electrode increases which may lead to the decline of output power performance. When it is required to add a compound of a rare earth element in the form of powder to a hydrogen absorbing alloy powder, the effect of the size of particles of the added compound upon the performance of the resulting electrode has been neglected, and a rare earth element compound in the form of coarse particles have been used. This probably explains the reason why addition of a particulate compound of a rare earth element did not produce a satisfactory result as expected.
For example, to be used for a power supply of an HEVs, a battery preferably has an output power performance of producing 1400 W/kg or more at 25° C. The output power performance at 25° C. of a conventional cylindrical nickel metal-hydride battery, however, is as low as 1000 W/kg. As shown in FIG. 4, a conventional cylindrical nickel metal-hydride battery comprises a lid which serves as one terminal (positive electrode terminal) out of the two terminals (the lid comprises a knob-like cap 6, a sealing plate 0, and a valve 7 provided in a space enclosed by the cap 6 and sealing plate 0; a gasket 5 is attached to the periphery of sealing plate 0; and the open end of a cylindrical container 4 with a bottom is folded tightly around the periphery of the lid so that the lid and the end of container are brought into air-tight and contact via gasket 5), wherein an upper current collecting plate 2 (current collecting plate to serve as the positive electrode) attached to the upper ends of a rolled electrode assembly 1 is connected to the sealing plate 0 via a ribbon-like current collecting lead 12 as shown in FIG. 5. In the manufacture of a conventional battery, one end of ribbon-like current collecting lead 12 is attached by welding to the internal surface of sealing plate 0; the other end of current collecting lead 12 is attached by welding to the upper current collecting plate 2; and then the lid is applied to the open end of container 4 to close the open end. Therefore, the current collecting lead 12 should have a curvature to produce an extra length. Because of this, the current collecting lead 12 connecting a welded point provided on the internal surface of sealing plate 0 with another welded point provided on the upper current collecting plate 2 usually has a length 6 to 7 times as large as the distance between sealing plate 0 and upper current collecting plate 2, which increases the electric resistance of the current collecting lead itself. This may act as one of the causes responsible for the degraded output power performance of a nickel metal-hydride battery.